Move some processing of Spotify data to the backend

[Noam-Bendelac]

I think it might be better to do things like turning ArtistObjectSimplified[] into a string of comma-separated artists, or getting the name and image url out of AlbumObjectSimplified in the backend instead of multiple places in the frontend. This will also help with mocking track data for the add state, like #70.

Things to possibly change:

• /api/playlists/:id/:
  • turning ArtistObjectSimplified[] into a string of comma-separated artists
  • getting the name and image url out of AlbumObjectSimplified
• processing all the search results data as described in Refactor API response type for search #70
• processing the /api/playlists response as described in Refactor /api/playlists endpoint #74 (done)
• processing the track length if Add track length to playlist view and search results #69 is implemented

All of these should be done for all applicable endpoints, like /api/playlists/:id, /api/playlists, /api/search, and maybe more.